This invention relates generally to apparatus and methods for controlling the frequency of light output from an optical signal source. This invention is particularly related to apparatus and methods for controlling the frequency of optical signals output from coherent light sources.
Stability in the optical frequencies input to optical fibers is a practical necessity in the development and implementation of sensing systems using optical fibers. Optical sensing systems may use semiconductor diode lasers or superluminescent diodes as light sources. Fiber optic rotation sensors have been used in broadband semiconductor light sources to reduce noise arising from backscattering in the fiber and for reducing errors caused by the optical Kerr effect. High precision fiber optic rotation sensors required a stable light source the wavelength because the scale factor of the sensor depends upon the source wavelength. For example, a navigation grade rotation sensor requires wavelength stability of about one part in 10.sup.6.
A wideband source such as a superluminescent diode (SLD) or a narrower source such as a single or multimode laser diode needs frequency stabilization in order to be suitable as an optical source for a Sagnac ring fiber optics rotation sensor.
The SLD provides a spectral linewidth sufficient to overcome unwanted phase errors due to coherent backscatter and the Kerr effect. The fractional linewidth should be between 10 and 1000 parts per million (ppm). The frequency stability of the centroid of the source spectral distribution should be several ppm to meet scale factor stability and linearity requirements. Therefore, source width should be minimized within the constraints of coherent backscatter and Kerr effect errors to enhance scale factor linearity. The fractional linewidth should approach the lower portion of the 10 to 1000 ppm range to minimize unwanted errors in scale factor due to changes in the source spectral distribution over time.
There are at least three groups of laser diodes that are classified according to structure. These are homostructure, single heterostructure and double heterostructure diode lasers.
The simplest diode lasers are called homostructure lasers because they are made of a single semiconductor material. A homostructure laser diode may comprise, for example, regions of n-type and p-type gallium arsenide. Electrons injected from the n-region into the p-region combine with holes, or positive charge carriers, to emit laser light. All laser diodes include two polished parallel faces that are perpendicular to the plane of the junction of the p-type and n-type regions. The emitted light reflects back and forth across the region between the polished surfaces and, consequently is amplified on each pass through the junction.
A typical single heterostructure semiconductor laser includes an additional layer of aluminum gallium arsenide, in which some of the gallium atoms in the gallium arsenide have been replaced by aluminum atoms. The aluminum gallium arsenide layer stops the injected electrons, thereby causing the emission of a higher intensity laser light than ordinarily occurs with a homostructure diode laser.
A typical double heterostructure semiconductor laser includes three layers of gallium arsenide separated by two layers of aluminum gallium arsenide. Preselection of either n-type or p-type materials causes further increases of the intensity of the emitted laser beam.
The wavelength of the light emitted from a laser diode varies as a function of the operating temperature and the injection current applied. Effective use of a laser diode as a light source in an optical rotation sensor requires an output of known wavelength. In fiber optic rotation sensing applications, the frequency stability should be about .DELTA.f/.sub.f =10.sup.6, and the light source should be held at a constant temperature.
Superluminescent diodes used as light sources in fiber optic rotation sensors typically have excessive fractional linewidths of about 10,000 ppm. They also have operating lifetimes of about 100 hours and provide about 500 .mu.W or less optical power into an optical fiber. SLD's have linewidth to frequency stability ratios of about 10,000 and require relatively high injection currents that typically exceed 100 mA. As a result, the short operating lifetime and excessive linewidths make SLD's unacceptable for fiber optic rotation sensors, which should operate reliably for thousands of hours without source replacement.
Single mode laser diodes have the characteristic that modulation of the injection current produces simultaneous amplitude and frequency modulations of the power output. The amplitude modulation has a modulation depth that approaches 100%. Periodic AM modulation at kilohertz or megahertz rates from below or near threshold to a high peak power can produce an output with a continuous spectral distribution exceeding 20 Ghz. It is possible to produce a chirp frequency modulation of the output frequency that exceeds 20 GHz, which is equivalent to a 50 ppm fractional linewidth at a wavelength .gamma.=820 nm. Modulation with a pseudo-random noise source of appropriate spectral density can produce an output with a desired spectral distribution and linewidth. Thus single mode laser diodes have the advantages of providing power inputs to an optical fiber in the range of 1-5 mW, long operating lifetime that exceed 10,000 hours, and a linewidth to frequency stability ratio that is adjustable over a range of about 10 to 100.
Multimode laser diodes have adjustable fractional linewidths that are dependent on the number of longitudinal modes that lase. For a five mode laser, the fractional linewidth may be about 1000 ppm at .gamma.=820 nm, which corresponds to a wavelegth change .DELTA..gamma.=0.2 nm. Injection current modulation at kilohertz or megahertz rates can smear the discrete longitudinal to produce a continuous or quasi-continuous spectral distribution over a fractional linewidth of 1000 ppm. Multimode laser diodes typically provide high power inputs in the range of about 1-10 mW into optical fibers, have operating lifetimes that typically exceed 10,000 hours and have a linewidth to frequency stability ratio in the range of about 100 to 1000.
Some familiarity with polarization of light and propagation of light within an optical fiber will facilitate an understanding of the present invention. Therefore, a brief description of the concepts used to describe the propagation and polarization of a light wave in a fiber is presented.
An optical fiber comprises a central core and a surrounding cladding. The refractive index of the cladding is less than that of the core. The diameter of the core is so small that light guided by the core impinges upon the core-cladding interface at angles less than the critical angle for total internal reflection.
It is well-known that a light wave may be represented by a time-varying electromagnetic field comprising orthogonal electric and magnetic field vectors having a frequency equal to the frequency of the light wave. An electromagnetic wave propagating through a guiding structure can be described by a set of normal modes. The normal modes are the permissible distributions of the electric and magnetic fields within the guiding structure, for example, a fiber optic waveguide. The field distributions are directly related to the distribution of energy within the structure.
The normal modes are generally represented by mathematical functions that describe the field components in the wave in terms of the frequency and spatial distribution in the guiding structure. The specific functions that describe the normal modes of a waveguide depend upon the geometry of the waveguide. For an optical fiber, where the guided wave is confined to a structure having a circular cross section of fixed dimensions, only fields having certain frequencies and spatial distributions will propagate without severe attenuation. The waves having field components that propagate with low attenuation are called normal modes. A single mode fiber will propagate only one spatial distribution of energy, that is, one normal mode, for a signal of a given frequency.
In describing the normal modes, it is convenient to refer to the direction of the electric and magnetic fields relative to the direction of propagation of the wave. If only the electric field vector is perpendicular to the direction of propagation, which is usually called the optic axis, then the wave is the to be a transverse electric (TE) mode. If only the magnetic field vector is perpendicular to the optic axis, the wave is a transverse magnetic (TM) mode. If both the electric and magnetic field vectors are perpendicular to the optic axis, then the wave is a transverse electromagnetic (TEM) mode.
None of the normal modes require a definite direction of the field components. In a TE mode, for example, the electric field may be in any direction that is perpendicular to the optic axis. The direction of the electric field vector in an electromagnetic wave is the polarization of the wave. In general, a wave will have random polarization in which there is a uniform distribution of electric field vectors pointing in all directions permissible for a given mode. If all the electric field vectors i a wave point in only a particular direction, the wave is linearly polarized. If the electric field consists of two orthogonal electric field components of equal magnitude and a phase difference of 90.degree., the electric field is circularly polarized, because the net electric field is a vector that rotates around the propagation direction at an angular velocity equal to the frequency of the wave. If the two linear polarizations are unequal or have a phase difference other than 90.degree., the wave has ellipical polarization. In general, any arbitrary polarization can be represented by the sum of two orthogonal linear polarizations, two oppositely directed circular polarizations or two counter rotating elliptical polarizations that have orthogonal major axes.
The boundary between the core and cladding is a dielectric interface at which certain well-known boundary conditions on the field components must be satisfied. For example, the component of the electric field parallel to the interface must be continuous. A single mode optical fiber propagates electromagnetic energy having an electric field component perpendicular to the core-cladding interface. Since the fiber core has an index of refraction greater than that of the cladding and light impinges upon the interface at angles greater than or equal to the critical angle, essentially all of the electric field remains in the core by internal reflection at the interface. To satisfy both the continuity and internal reflection requirements, the radial electric field component in the cladding must be a rapidly decaying exponential function. An exponentially decaying electric field is usually called the "evanescent field".
The velocity of an optical signal depends upon the index of refraction of the medium through which the light propagates. Certain materials have different refractive indices for different polarizations. A material that has two refractive indices is said to be birefringent. The polarization of the signal propagating along a single mode optical fiber is sometimes referred to as a mode. A standard single mode optical fiber may be regarded as a two mode fiber because it will propagate two waves of the same frequency and spatial distribution that have two different polarizations. Two different polarization components of the same normal mode can propagate through a birefringent material unchanged except for a velocity difference between the two polarizations.
In summary, any polarized light can be represented by two circularly polarized waves having proper phase and amplitude. Alternatively, the light could be represented by either elliptically rotating components or by perpendicular linearly polarized components of the electric field.
There are a number of birefringent materials. Depending on their structure and orientation to the light propagating through it, certain crystals are circularly birefringent; some crystals are linearly birefringent. Other types of crystals, for example quartz, can have both circular birefringence and linear birefringence so as to produce elliptical birefringence for a light wave propagating in a properly chosen direction.